Diphtheria and pseudomonas toxins are virulence factors produced by Corynebacterium diphtheriae and Pseudomonas aeruginosa respectively. These distinct protein toxins both inhibit protein systhesis by inactivating mammalian cell elongation factor 2. The manner by which toxins bind to and enter cells and reach their substrate is not known. Preliminary evidence in our laboratory suggests that pseudomonas toxin enters cells by receptor mediated endocytosis, a process previously associated only with internalization of physiologically important molecules such as peptide hormones, low density lipoprotein and lysosomal enzymes. The specific objectives of the research are: 1) To determine if pseudomonas and diphtheria toxins are internalized by toxin sensitive cells via receptor mediated endocytosis. 2) To determine how toxins are processed by toxin sensitive cells and how they ultimately reach their cytoplasmic substrate, elongation factor 2. 3) To determine if cellular resistance or sensitivity to pseudomonas and diphtheria toxins is governed by the presence or absence of ligand-receptor interactions. Biochemical and electron microscopic studies will be carried out in parallel. Agents which are known to alter receptor mediated endocytosis and intralysosomal protein degradation will be examined for their effect on toxin activity. Toxin entry and processing by toxin sensitive and resistant cells will be compared.